The UV-sensitive mutations uvrD and uvrE map closely together on the chromosome of Escherichia coli. We have confirmed our previous results which contradict that of others showing that the two mutations result in distinctive phenotypes. Studies of partial diploids indicate that the uvrD3 mutation is partially dominant. UV- and methyl methanesulfonate survival and mutation frequencies will be determined in uvrD/uvrE and uvrD/recL partial diploids. Several mutator mutations that are linked to cysC will be examined by complementation tests to determine if they are in the same gene. Among these are mutations induced by the insertion of transposable element Tn5. These insertion mutations will be used to look for suppressors of the mutator phenotype. Work will continue on mutations blocking the growth of phage P1. One such mutation may interact with mutS. We have examined 130 natural strains of E. coli for the mutator phenotype and will continue this survey.